Retrieving data from replaced entries

[domenic]

A nice feature of the navigation API is that (unless we implement #9) data is almost never "lost". This solves a lot of problems we hear about today where history.state is unreliable, or there's no way to easily know what your previous history entry was without keeping a side table, etc.

However, there is one place in the current design where data can be lost: if you use navigation.navigate(url, { replace: true }), history.replaceState(), or location.replace(), the current navigation API history entry data disappears.

I say "kind of" because object references get replaced, not updated. So you could have a reference to the old NavigationHistoryEntry object, which would allow you to access url and getState(). (But it's no longer in appHistory.entries().)

We could pretty easily keep these replaced NavigationHistoryEntrys around and expose them. For example:

• Each app history entry could have a replaced property that is null, or the entry it replaced. (This allows potentially chaining, e.g. navigation.currentEntry.replaced.replaced.replaced.) This lets you access them forever. (Is this a problem for memory usage?)

• The currententrychange event could have a replaced property which contains the replaced NavigationHistoryEntry. This lets unrelated parts of the code (i.e., not the code doing navigation.navigate(, { replace: true })) grab important data as necessary.

[tbondwilkinson]

The only other case where they got lost is if they're in the forward stack, or in most browsers if it's evicted by the back stack growing too long (I believe the limit is 40). I think the dispose event helps to find out when this happens though, so it's not a major concern.

Another possible solution is to identify in the dispose event that it was a replace, rather than an evict, which would allow you to grab a handle on the entry (and state) that's replacing yours.